Cd in neutron shield

[Lester Slaback <Lester.Slaback@NIST.GOV>]

The question is - what is the reason for the Cd in this neutron shield?
 > OUTSIDE WORLD
 > 0.020 inches of cadmium
 > 0.062 inches of steel
 > 6.0 inches of paraffin
 > 0.062 inches of steel
 > 4.0 inches of air
 > PuBe source (5 Ci)

It results in a rather minor dose reduction, i.e., converting the thermal 
neutron fraction to high energy Cd photons is only about a 30% dose 
reduction for that leakage fraction.  If the intent is dose reduction then 
a Li or boron compound would be a better choice.

If the intent is just to remove the thermal neutron leakage for neutron 
background control then it does not matter.

The prompt gammas produced by the Fe give more dose per neutron captured 
but the cross-section is so much lower that the fraction captured in the Fe 
is not an issue.  But if those capture lines are an issue then the Cd 
should be placed on each side of the paraffin between the paraffin and the 
steel.

 From a dose point of view the H capture in the paraffin (resulting in 2.2 
MeV photons) is probably controlling in this geometry.  Typically a boron 
compound is mixed with the paraffin to competitively capture the thermals 
thereby producing 0.5 MeV photons, which are also easier to shield.  Then 
the Cd is not needed.
Disclaimer:  the above are the personal musings of the author, and do not 
represent any past, present, or future position of NIST, the U.S. 
government, or anyone else who might think that they are in a position of 
authority.
Lester Slaback, Jr.  [Lester.Slaback@NIST.GOV]
NBSR Health Physics
Center for Neutron Research
NIST
100 Bureau Dr.  STOP 3543
Gaithersburg, MD  20899-3543
301 975-5810 voice
301 921-9847 fax

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